Matthias Pfannerstill

Dynamic Modelling of Land Use Change Impacts on Nitrate Loads in Rivers

Agricultural land use practices were found to have a great impact on river water quality. Highly variable boundary conditions such as seasonal weather influence or market impacts lead to dynamic changes in the use of agricultural crops. In rural areas with a huge share of agricultural land use, these dynamics are accompanied with changes of river water quality. To quantify these impacts for the future, it is common practice to apply scenario simulations with eco-hydrological catchment models. For this, it is essential to represent spatially and temporal variations in agricultural crops. In this study, we present a dynamic modelling approach of spatial agricultural crops distribution and its impacts on nitrate loads in rivers at the catchment scale. Area proportions of crops are reproduced in a data-based approach according to the current land use situation in the catchment. Using this spatial crop distribution, the eco-hydrological model SWAT is calibrated for discharge and nitrate time series. Within scenario simulations, the spatial crop distribution is updated dynamically for each year, while non-agricultural land use areas remain constant. The dynamic approach is compared with a static land use scenario, in which the land use information is changed only once immediately at the beginning of the scenario period. Good model results were realised for discharge and nitrate loads in the calibration and validation period. The evaluation of the land use change scenarios for the period from 2021 to 2030 revealed changes in nitrate loads. The analysis of the dynamic land use update illustrates an increase in the changes between the scenarios within the modelling period. Consequently, the presented approach allows the quantification of nutrient dynamics that are related to dynamic changes of land use during the simulation period. We conclude that this study shows how the dynamic modelling leads to a more realistic temporal development of land use change and its impacts on nitrate.

How to Constrain Multi-Objective Calibrations of the SWAT Model Using Water Balance Components

Accurate discharge simulation is one of the most common objectives of hydrological modeling studies. However, a good simulation of discharge is not necessarily the result of a realistic simulation of hydrological processes within the catchment. We propose an evaluation framework that considers both discharge and water balance components as evaluation criteria for calibration of the Soil and Water Assessment Tool (SWAT). In this study, we integrated average annual values of surface runoff, groundwater flow, and evapotranspiration in the model evaluation procedure to constrain the selection of good model runs for the Little River Experimental Watershed in Georgia, United States. For evaluating water balance and discharge dynamics, the Nash-Sutcliffe efficiency (NSE) and percent bias (PBIAS) were used. In addition, the ratio of root mean square error and standard deviation of measured data (RSR) was calculated for individual segments of the flow duration curve to identify the best model runs in terms of discharge magnitude. Our results indicate that good statistics for discharge do not guarantee realistic simulations of individual water balance components. Therefore, we recommend constraining the ranges of water balance components to achieve a more realistic simulation of the entire hydrological system, even if tradeoffs between good statistics for discharge simulations and reasonable amounts of the water balance components are unavoidable. Editors note: This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.

Lentic small water bodies: Variability of pesticide transport and transformation patterns

Lentic small water bodies have a high ecological potential as they fulfill several ecosystem services such as the retention of water and pollutants. They serve as a hot spot of biodiversity. Due to their location in or adjacent to agricultural fields, they can be influenced by inputs of pesticides and their transformation products. Since small water bodies have rarely been part of monitorings/campaigns up to now, their current exposure and processes guiding the pesticide input are not understood, yet. This study presents results of a sampling campaign of 10 lentic small water bodies from 2015 to 2016. They were sampled once after the spring application for a pesticide target screening, before autumn application and three times after rainfall events following the application. The autumn sampling focused on the herbicides metazachlor, flufenacet and their transformation products - oxalic acid and - sulfonic acid as representatives for common pesticides in the study region. The concentrations were associated with rainfall before and after application, characteristics of the site and the water bodies, physicochemical parameters and the applied amount of pesticides. The key results of the pesticide screening in spring indicate positive detections of pesticides which have not been applied for years to the single fields. The autumn sampling showed frequent occurrences of the transformation products, which are formed in soil, from 39% to 94% of all samples (n=71). Discharge patterns were observed for metazachlor with highest concentrations in the first sample after application and then decreasing, but not for flufenacet. The concentrations of the transformation products increased over time and revealed highest values mainly in the last sample. Besides rainfall patterns right after application, the spatial and temporal dissemination of the pesticides to the water bodies seems to play a major role to understand the exposure of lentic small water bodies.

Modelling of hydrological processes in snowmelt-governed permafrost-free catchments of the Western Siberian Lowlands

To date, no examples of small- to meso-scale hydrological simulations exist in the southern part of the Western Siberian lowlands, despite intensive agriculture and high vulnerability to climate change. We propose a first simulation approach in which we assess the importance of surface and groundwater processes on hydrological model performance. Therefore, we simulated three catchments, using four different model setups incorporating different landscape characteristics and processes. An objective calibration and comparison framework was applied to assess the different setups which reached very diverse performance: the setups where physically-based surface retention is considered, showed slightly more realistic surface runoff driven peak flows and the setups with a more complex groundwater concept improved the depiction of surface runoff, the recession phase and the contributing baseflow significantly. The best performing, most complex setup was used to assess the prevailing hydrological processes of the lowland with its cold, continental climate in more detail.